A touch panel device includes various touch panel devices. Among these touch panel devices, an electrostatic capacitive touch panel device is highly convenient because a user can operate it with, e.g., a finger of the user or a mere electrically-conductive pen without any special pen.
However, in a case where a user who is being subjected to electromagnetic noise (from, e.g., a fluorescent light, a switching power source, an electronic device, or a radio) touches a touch panel device via an indicator (a finger of the user or an electrically-conductive pen), the electromagnetic noise enters the touch panel device. The electromagnetic noise which is gotten by a human body etc. and enters the touch panel device via the indicator is called “external noise”. The touch panel device which the external noise has entered will probably detect an incorrect touch position.
Some conventional touch panel devices have a system for removing external noise.
Patent Literature 1 describes an electrostatic capacitive touch panel device which avoids being affected by external noise. This touch panel device detects degree of variation in level signal outputted from a reception section, and carries out a sampling at timing when the degree is small. This touch panel device determines a sampling timing of a sample holding section or a frequency of a driving signal so that the degree is reduced. This makes it possible to obtain a stable output signal which is not affected by external noise.
Patent Literature 2 describes an electrostatic capacitive touch panel device which avoids being affected by external noise. This touch panel device changes a frequency of one of driving signals to another frequency in a case where the frequency of the driving signal equals to a frequency of external noise. This differentiates, from the frequency of the external noise, a frequency of a voltage signal which frequency equals to the frequency of the driving signal. In addition, the external noise is removed by use of a band pass filter for filtering a component of a frequency different from the frequency of the driving signal.
Patent Literature 3 describes a touch panel device which employs a surface acoustic wave method. This touch panel device determines, from a reception signal based on a burst wave of the surface acoustic wave method, whether or not noise exists. In a case where the touch panel device determines that noise exists, the touch panel device does not detect an object on the basis of the reception signal.